Device for cutting a structure in a motor vehicle

ABSTRACT

The technical field of the invention is that of motor vehicle safety involving devices for cutting a structure such as, for example, an instrument panel, so as to actuate a safety device.  
     The object of the present invention relates more specifically to a device ( 1 ) for cutting a structure in a motor vehicle comprising a linear pyrotechnic charge ( 5 ) placed inside a flattened deformable tube ( 6 ), combustion of the said charge ( 5 ) bringing about deformation of the tube ( 6 ) which increases its cross section, the said deformation causing the structure ( 3 ) to break.  
     The main characteristic of this device is that the combustion rate of the pyrotechnic charge ( 5 ) is less than 3000 m/s.

The technical field of the invention is that of motor vehicle safety involving devices for cutting a structure such as, for example, an instrument panel, so as to actuate a safety device. Cutting devices according to the invention are particularly suited to the application of airbags in a motor vehicle.

Devices for cutting a structure have already been the subject of several patents.

Mention may be made of patent EP 0 273 061 which describes a separation joint between two structures and involves a linear explosive charge confined in a flattened tube. The two structures fit into each other and ignition of the explosive charge brings about expansion of the tube which then deforms one of the two structures bringing about their separation. The explosive charge is not specifically described but the description reveals some indications that suggest that it consists of a detonating charge.

Similarly, the separating device disclosed in U.S. Pat. No. 6,170,400 or in U.S. Pat. No. 5,372,071 employs a detonating fuse, ignition of which will bring about deformation of the tube in which it is housed so as to push back a piston in order to separate two sub-assemblies.

Finally, mention may be made of U.S. Pat. No. 5,478,106 which describes an opening system for deploying an airbag. This opening system comprises a deformed tube having a tapered part and in which a linear pyrotechnic charge is housed. Ignition of the said charge brings about expansion of the tube, the tapered part of which comes into contact with the structure in order to cut it.

Cutting devices according to the invention employ deflagrating fuses housed in flattened tubes and ignition of these causes the cross section of the said tubes to expand so as to cut a structure. Deflagrating fuses have slower reaction rates than those of detonating fuses and the associated pyrotechnic effects are consequently less violent and in particular the risks of projection of splinters and the emission of shock waves in the air outside the said devices become negligible. Cutting devices according to the invention are particularly suited to motor vehicles in as much as they are reliable and efficient while exhibiting an increased safety level by limiting the intensity of the reaction and hence the possibilities of physical injuries.

The object of the present invention relates to a device for cutting a structure in a motor vehicle comprising a linear pyrotechnic charge placed inside a flattened deformable tube, combustion of the said charge bringing about deformation of the tube which increases its cross section, the said deformation causing the structure to break, characterized in that the combustion rate of the pyrotechnic charge is less than 3000 m/s.

In this patent application, the expressions “deflagrating charge” and “charge of which the combustion rate is less than 3000 m/s” are equivalent.

This equivalence is desirable since although deflagration is defined by a longer and less violent reaction than a detonation, it remains relatively poorly defined as regards the value of certain parameters characterizing it.

The combustion rate of the pyrotechnic charge preferably lies between 1000 m/s and 2000 m/s.

The deflagrating charges selected for cutting devices according to the invention are sufficiently lively to bring about cutting of a structure in a motor vehicle, without exhibiting difficulties associated with the use of a detonating charge such as for example the risks of the cutting device breaking with emission of undesirable pyrotechnic effects such as a shock wave in the air and the projection of splinters.

Advantageously, the pyrotechnic charge comprises a plurality of strands such as, for example, those described in U.S. Pat. No. 4,220,087.

The tube is preferably metallic.

Advantageously, the said tube has a good ability to deform plastically.

Preferably, the tube is closed at each of its ends by a stopper so as to contain the pyrotechnic effects inside the said tube.

Indeed, since the requirements for motor vehicle safety are particularly high, it is recommended to minimize the risks of the propagation of untimely effects outside the cutting device.

The tube is preferably made of aluminium.

Advantageously, a space exists between the pyrotechnic charge and the tube, the said space being packed with a foam made of an elastomer intended, on the one hand, to hold the said charge in the centre of the said tube, and, on the other hand, to increase the pressurization of the said tube. In this way, this makes it possible, with a small pyrotechnic charge and therefore with a small quantity of gas produced, to maximize stresses, even in strong tubes, so that they recover their original cylindrical shape.

Preferably, a device for igniting the pyrotechnic charge comprising an igniter is placed at one of the ends of the tube.

Advantageously, the flattened tube is housed in a groove of the structure having a U-shaped cross section, the bottom of the groove including a secondary groove and the tube being held by means of a closure clip.

Preferably, the linear density of the pyrotechnic charge is less than 4 g/m.

Cutting devices according to the invention have the advantage of not being dangerous for persons, whether they are functioning normally or erratically, in as much as the effects produced remain of low intensity. Moreover, they are reliable, since ignition of such compositions is perfectly controlled. Finally, on account of the great diversity of deflagrating compositions that can be used, cutting devices according to the invention can be adapted to a large number of configurations associated in particular with the thickness of the structure to be cut, with the material constituting the said structure and with the surface to be cut.

A detailed description of a preferred embodiment of the invention is given hereinafter with reference to FIGS. 1 and 2.

FIG. 1 is a view in lateral axial section of a cutting device according to the invention, positioned in relation to the structure to be cut and not having yet functioned.

FIG. 2 shows the cutting device of FIG. 1 after operation.

With reference to FIG. 1, the cutting device 1 according to the invention is inserted into a groove 2 of the structure 3 to be cut, the said groove 2 having a U-shaped cross section, the bottom of which includes a secondary v-shaped groove so that the thickness of the structure 3 in the region of the said secondary groove 4 is at a minimum. A linear pyrotechnic charge 5 consisting of three strands is housed in the centre of a hollow deformable aluminium tube 6, the said tube 6 being flattened so as to have an elongate section possessing two flat sides that are parallel to each other and connected to each other by two segments in the arc of a circle, the said charge 5 being in contact with the two flat sides of the tubes 6. The space existing between the pyrotechnic charge 5 and the inner wall of the tube 6 is packed with an elastomeric foam 7 to assist in holding the said charge 5 in the centre of the said tube 6. The tube 6 occupies the groove 2 without any lateral play, in other words each of the two flat sides of the tube 6 is in contact with the walls of the groove 2. The groove 2 is made in a bead of the structure 3 and the tube 6 is held in the groove 2 by means of a closure clip 8 covering the said groove 2. The tube 6 in the groove 2 is in contact at the same time with the bottom of the said groove 2 and the closure clip 8. An ignition device comprising a single igniter is placed close to the tube 6. Two stoppers close the ends of the said tube 6.

With reference to FIGS. 1 and 2, the method of operating the cutting device according to the invention is as follows.

Ignition of the pyrotechnic charge 5 brings about the production of gas in the tube 6 which is then subjected to a high internal pressure. The tube 6 reacts by deforming so as to adopt a circular cross section.

This expansion causes the structure 3 to break in the region of its smallest thickness created by the secondary groove 4, as well as the closure clip 8. The structure is, thus cut and all the pyrotechnic effects remain contained in the tube 6. 

1. Device for cutting a structure in a motor vehicle comprising a linear pyrotechnic charge placed inside a flattened deformable tube, combustion of the said charge brining about deformation of the tube which increases its cross section, the said deformation causing the structure to break, characterized in that the combustion rate of the pyrotechnic charge is less than 3000 m/s.
 2. Cutting device according to claim 1, characterized in that the pyrotechnic charge comprises a plurality of strands.
 3. Cutting device according to claim 1, characterized in that the tube is metallic.
 4. Device according to claim 1, characterized in that the tube is closed at each of its ends by a stopper so as to contain the pyrotechnic effects inside the said tube.
 5. Device according to claim 3, characterized in that the tube is made of aluminum.
 6. Device according to claim 1, characterized in that a space exists between the pyrotechnic charge and the tube, the said space being packed with a foam made of an elastomer.
 7. Device according to claim 1, characterized in that a device for igniting the pyrotechnic charge comprising an igniter is placed at one of the ends of the tube.
 8. Device according to claim 1, characterized in that the flattened tube is housed in a groove of the structure having a U-shaped cross section, the bottom of the groove including a secondary groove and the tube being held by means of a closure clip.
 9. Device according to claim 1, characterized in that the linear density of the pyrotechnic charge is less than 4 g/m.
 10. Cutting device according to claim 2, characterized in that the tube is metallic 